1. Field of the Invention
The present invention relates to a camera that switches aperture sizes automatically according to subject brightness, more particularly to a camera that changes threshold levels of the subject brightness for changing the aperture size between a flash photography and a daylight photography.
2. Background Arts
As is well known, there is such a camera of a simple exposure mechanism as a lens-fitted photo film unit previously loaded with a photo film. Since the lens-fitted photo film unit is a kind of single-use camera, it is desirable to simplify its structure and lower cost for manufacturing. And a lens-fitted photo film unit where a flash device is built in is also well known. When a flash charge switch thereof is turned on, the flash device emits light synchronized with a shutter so as to perform the flash photography.
In the recent years, at users"" demand for higher image quality, research and development have been made to produce a lens-fitted photo film unit that is provided with an automatic stop change device for changing stop apertures automatically in accordance with subject brightness. One case of the automatic stop change device is constituted of a stop plate or blade provided with a small opening (hereinafter referred to as xe2x80x9csmall stop aperturexe2x80x9d) and a stop control circuit for inserting the stop plate in a photographic light path. If the subject brightness is below a threshold, the stop plate is retired from the photographic light path, so a frame of a photo filmstrip is exposed through a fixed stop aperture. The fixed stop aperture is formed through a shutter cover in an example, and a size thereof is larger than that of the small stop aperture. Although such a fixed aperture-stop is generally called an open aperture-stop, yet the fixed stop aperture is referred to as xe2x80x9cnormal stop aperturesxe2x80x9d in the present specification. When the subject brightness is above the threshold, the stop control circuit is operated when the shutter release button is depressed halfway, to set the stop plate on the photographic light path. A shutter mechanism is operated when the shutter release button is fully depressed, so that light through the small stop aperture enters the photo filmstrip. As the aperture sizes are automatically switched correspondingly to the subject brightness in the automatic stop change device, an available range of the subject brightness can be enlarged.
However, in the automatic stop change device as mentioned above, the threshold level for switching the stop apertures is fixedly determined, thus there are a first subject brightness range where the normal stop aperture is selected, and a second subject brightness range where the small stop aperture is selected. However, in a daylight photography where the flash device is unused, it is possible to take photographs at permissible exposure conditions with the small stop aperture even while the subject brightness is in the first subject brightness range, if it is on higher brightness side of the first range. Yet within the first subject brightness range, as the normal stop aperture is used, a depth of field becomes smaller. That can result an insufficient focusing at long distance photography. Also in a flash photography where the flash device is used, it is possible to take photographs at permissible exposure conditions with the normal stop aperture even while the subject brightness is in the second subject brightness range, if it is on lower brightness side of the second range. Yet within the second brightness range, as the small stop aperture is used, the background is under-exposure. In this way, the conventional automatic stop change device cannot control exposure suitably for the subject brightness around the threshold level.
In view of the foregoing, an object of the present invention is to provide a camera that makes it possible to change the aperture size suitably for photographic condition.
Another object of the present invention is to provide a camera with a simple structure for changing the aperture size.
A further object of the present invention is to provide a camera that makes it possible to increase exposure amount of a background in a flash photography, and have a greater depth of field in a daylight photography within a predetermined subject brightness range.
The above and other objects can be accomplished by changing amount of light entering a photo sensor in response to operation of a flash charge switch, thereby to change threshold levels of the subject brightness for changing the aperture size. A signal from the photo sensor is transmitted to a comparison circuit. In the comparison circuit, if output of a photometric element is above a predetermined value, a stop plate inserts a photographic light path to take photography by use of a small stop aperture.
In the embodiments of the present invention, a first light guide and a second light guide are provided and selectively used for the purpose of transmitting light from a photometric window to the photo sensor. The first light guide is selected for the daylight photography (in which a flash device is unused). As the first light guide has a shorter distance from an incident end to the photometric window, a larger amount of light enters the incident end. The second light guide is selected for the flash photography. As the second light guide has a longer distance from an incident end to the photometric window, a smaller amount of light enters the incident end. In the first light guide, if the subject brightness is above S1, the output from the photometric element becomes above the predetermined value, and the stop plate inserts the photographic light path. As the stop plate has the small stop aperture, the exposure amount is restricted. In the second light guide,if the subject brightness is above S2 (S1 less than S2), the output from the photometric element becomes above the predetermined value and the stop plate inserts the photographic light path. Therefore, the threshold level of the subject brightness becomes S1 in the daylight photography, and the threshold level of the subject brightness becomes S2 in the flash photography.
Either the first light guide or the second light guide is selected in response to the flash charge switch. And two light guides that are different from each other in sectional area of the incident end may be used for the purpose of changing the amount of light inserting the respective incident end.
In the present invention, the threshold of the subject brightness that are reference values for changing the stop apertures are changed by controlling the amount of incident light on the photo sensor in response to the operation of the flash charge switch, so that the stop apertures can be changed by means of a simple structure to the suitable size for the photographic condition around those subject brightness levels where the aperture size is to be changed.
Among all, since there are a plural number of the light guides which are different from each other in the amount of the incident light, and one of the light guides is selected in response to the flash charge switch, to guide the light from the photometric window to the photo sensor, it is easy to change the threshold levels of the subject brightness. Also, if two light guides whose distances from the photometric window to the incident end are respectively different are used, photometric ranges are changed as well as the amount of light. Thereby, it is possible to change the aperture size on the basis of averaged brightness measuring for measuring brightness of a whole field to be taken and center-weighted measuring for measuring the center of the field particularly.